Periplasmic membrane-bound system for detecting protein-protein interactions

ABSTRACT

The present invention relates to: 
     (1) a fusion protein having a dimerizing domain with or without a ligand-binding region and toxR DNA-binding and hydrophobic transmembrane regions; 
     (2) host cells comprising the fusion protein and a nucleic acid molecule having a reporter gene operatively linked to the ctx operon, wherein dimerization (ligand-dependent or -independent) is signaled by expression of the reporter gene; 
     (3) a nucleic acid molecule coding for the fusion protein; 
     (4) an expression vector comprising a coding region for the fusion protein; 
     (5) a process for detecting dimer formation (ligand dependent or ligand independent) of the fusion protein, which comprises treating a culture of the host cells with a ligand, ligand mimetic, or dimerization inhibitor, and screening for expression of the reporter gene. 
     The present invention can be used to generate a signal from a variety of ligand-binding domains, allowing ligand binding to be indicated by a simple colorimetric test or antibiotic resistance. The fusion proteins could include therapeutically relevant domains, so that biologically pertinent interactions can be indicated by a readily measurable signal.

FIELD OF THE INVENTION

This invention relates to assays for protein-protein interactions,fusion proteins, and host cells modified to comprise such proteins.

BACKGROUND OF THE INVENTION

Information about the extracellular environment is communicated to theinside of a cell by so-called signal transduction systems. Schlessinger,J., TIBS 13: 443-7 (1988); Stock, J. B., et al., Microbio Rev.53:450-490(1989). Several classes of signal transducers are able to "transmit" asignal without internalization of the primary signal. Such signaltransmission may result from binding of a ligand to a membrane-boundreceptor, inducing a conformational change in the receptor.

One such signal transducer is the toxR-ctx system in Vibrio cholerae.Ctx is the gene for cholera toxin. ToxR protein is a transmembraneprotein with a DNA-binding domain at its amino terminus, a stronglyhydrophobic transmembrane domain, and an environmentally regulated,dimer-forming domain at its carboxyl terminus. Miller, V. L., et al.Cell 48:271-279 (1987). According to current beliefs, the hydrophobictransmembrane region directs the carboxyl terminus to the periplasmwhile the amino terminus remains in the cytoplasm. A change inosmolarity or temperature in the periplasm is believed to effect thecapacity of the toxR protein to form dimers with its carboxyl terminus.The carboxyl dimers in turn modulate the ability of the amino terminusof toxR to bind to repetitive sequences in the ctx promoter and,thereby, modulate the ability of RNA polymerase to initiatetranscription at the promoter. Id.

To test the hypothesis that toxR protein is a membrane protein, Milleret al. fused toxR protein with alkaline phosphatase (phoA). Cell48:271-279 (1987). The phoA enzyme naturally exists as a dimer in theperiplasm and the phoA-toxR fusion protein was constitutively functionalwith respect to both phosphatase activity and toxR activation of ctx.This result demonstrated that toxR was able to direct alkalinephosphatase to the periplasm where it was active, and that thedimerization capacity of alkaline phosphatase could direct the toxRprotein to activate ctx transcription.

In a similar fusion experiment, Mekalanos et al, fused toxR to a phoAmutant that requires Zn²⁺ for dimerization and activity (personalcommunication). This fusion protein's phosphatase activity was dependenton Zn²⁺, as expected, but so also was activation. This result isconsistent with the signal transduction model for toxR action.

Chimeric signal transduction systems have been constructed inprokaryotic cells with different bacterial systems (Utsumi, J., etal.,Science 245:1246-1249), in tissue culture cells with differenteukaryotic hormone receptors (Riedel, H., et al., EMBO J. 8:2943-2954(1989)), and in tissue culture cells with a bacterial aspartate bindingdomain and the insulin receptor (Moe, G. E., .et al., Proc. Natl. Acad.Sci 86:5683-7 (1989)). To date, however, no such system has employed thetoxR receptor for V. cholerae.

SUMMARY OF THE INVENTION

The present invention relates to host cells comprising (a) atransmembrane fusion protein having (i) a periplasmic dimerizationdomain (e.g., a ligand-binding domain) and (ii) a toxR region having acytoplasmic toxR DNA-binding region and a hydrophobic toxR transmembraneregion, and (b) a nucleic acid molecule having a reporter geneoperatively linked to the ctx operon, wherein dimer formation, which mayor may not involve ligand binding, is signaled by expression of thereporter gene.

The present invention also relates to the aforementioned fusion protein,a corresponding nucleic acid molecule, and a corresponding expressionvector having an inducible promoter. Further, the invention includes aprocess for detecting dimerization (e.g., as a result of binding of aligand) of a membrane-bound protein, which comprises treating a cultureof the host cells of this invention with a ligand and screening forexpression of the reporter gene.

The present invention can be used to generate a signal from a variety ofligand-binding domains. Presence of a chosen ligand can be indicated byantibiotic resistance or a simple colorimeteric test for production ofan enzyme. The periplasmic regions of the transmembrane protein couldinclude therapeutically relevant domains, so that biologically pertinentinteractions can be indicated by a readily measurable signal. Suchtransmembrane fusion proteins should also contribute to ourunderstanding of dimerization domains, ligand-binding domains, proteinmembrane-spanning sequences, and signal transduction.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of the mechanism employed by thepresent invention. As shown in FIG. 1, ligand-binding protein LBP islocated within cell inner membrane IM and outer membrane OM. When thelevel expression of the toxR protein reaches a certain critical level,dimerization will ensue. Alternatively when a ligand L binds to theligand binding domain of a toxR fusion protein, it may causedimerization. In either instance, the dimerization induces aconformational change in the cytoplasmic domain of the wild type or toxRfusion protein which induces binding to the promoter region p of thereporter gene r. This DNA binding allows the RNA polymerase totranscribe the reporter gene, thus signaling dimerization which mayresult from either the spontaneous or ligand induced capacity of theperiplamic domain to asociate.

FIG. 2 shows control and fusion vectors. "LACp" denotes the lacpromoter; "AmpR", the ampicillin resistance gene;"Spec", thespectinomycin resistance gene; "LacZ," the β-galactosidase gene; "CTXp",the cholera toxin promoter; "CAT", the chloramphenicol transferase gene.(A) pTOX3. This wild-type ToxR control vector includes a toxR PCR clone(SEQ. ID. NO. 1) from genomic V. cholerae in expression vector pSPORT1.The toxR gene is placed under the control of the promoter, making itinducible by treatment with IPTG. The desired transductants areselectable by treatment with ampicillin. The ColEI origin is compatiblewith the pSC101 origin. (B) pTOX5. This toxR fusion vector includes atruncated toxR PCR clone (SEQ. ID. NO. 2) from genomic V. cholerae inthe vector pSPORT 1. pTOX5 allows the in-frame cloning of foreigndomains and may be used to generate families of fusions based onunidirectional deletion. The truncated pTOX5 does not activate the ctxpromoter and any signals found in chimeric proteins result from thefused domain's capacity to dimerize. (C) pCTX3. This reporter genevector includes the cholerae toxin promoter, ctx (SEQ. ID. NO. 3),PCR-amplified from p3083 (Infection and lmmunology, 58 pages 4142-4144,1990) and cloned in a novel promoterless lacZ vector. This constructexpresses β-galactosidase as a function of the dimerization of ToxR orToxR fusion proteins. The pSC101 origin is compatible with the ColEIorigin. The desired transformants may be selected by treatment withspectinomycin (D) pCTX4. This vector includes a promoterless CAT genefrom pCaMVCN inserted into the LacZ of pCTX3. Like the pCTX3 vector fromwhich it is derived, pCTX4 also provides spectinomycin selection andexpresses chloramphenicol acetyltransferase as a function ofdimerization of toxR or toxR fusion proteins.

FIG. 3 shows the results for the wild-type toxR gene, the truncated toxRfrom pTOX5, and a toxR in-frame gene fusion. The fusion in the exampleis to the leucine zipper dimerization domain of the yeast GCN4 protein,a well-studied dimerization domain representing an important control.(A) Co-transfection with lacZ reporter gene. For plasmids pTOX3, pTOX5,and pTOX5GCN4, this figure shows the amount of β-galactosidase produced(measured in Miller units) in the presence of zero (first bar), 50(second bar), and 200 (third bar) μM IPTG. These results show thatβ-galactosidase expression is dependent on the induction of the wildtype toxR gene of TOX3 by IPTG, and demonstrate that the truncated toxRproduct in pTOX5 fails to activate expression even following induction.The results noted for pTOX5-GCN4 demonstrate that a foreign dimerizationdomain can restore inducible activation. (B) Go-transfection with Theresults given in part B show the activation of the chloramphenicolresistance gene from pCTX,4 as expected for the strains expressing thewild-type toxR and chimeric toxR-GCN4 . The amount of chloramphenicolused is noted at the top of each column in μg/mL. "J4" denotes thestrain JM101 as transfected with pCTX4. "R" signifies chloramphenicolresistance; "S", sensitivity. Cells were grown in the presence of IPTGas noted in the materials and methods; when IPTG is omitted,chloramphenicol resistance is not observed.

FIG. 4 shows a method for generating unidirectional deletions, asapplied to a toxR-trkC fusion. TrkC is a member of the tyrosine kinaseclass of receptors and interacts with the nerve growth factor NT3 (Cell,66, pages 967-979, 1991). A full length trkC cDNA clone was placeddownstream of the truncated toxR gene in the fusion vector pTOX5 at theNotI cleavage site. Following digestion with SflI and NotI, this vectorwas treated with exonuclease III (ExoIII), which provides 5'-to- 3'directional deletions starting at the SfiI cleavage site. ExoIII-treatedplasmids were subsequently digested with mung-bean nucleus. Uponligation, this treatment results in a family of fusions between toxR andtrkC segments of varying length.

FIG. 5 shows the results for the toxR-trkC fusions prepared by theprocedure of FIG. 4. (A). βgalactosidase assay. The toxR activity ofvarious fusions was measured by β-galactosidase expression. Assays wereperformed in rich media (column LB) and minimal media (column min.) bothin the presence of (100 μM) and absence of IPTG. (B) sequencing of trkCportion. The table shown in part B reports the size (in base pairs) andthe boundaries of the sequence deleted in the fusion as determined bycomparing the sequence of the deletion mutant with that of the knowntrkC and toxR sequences. (C). Map of toxR-trkC fusions. The data in thetable in part B is expressed graphically with the hollow boxesindicating the base pairs deleted in the indicated fusion.

FIG. 6 shows the results obtained with fusions of HSV ICP35 and HIVintegrase to the toxR deletion of pTOX5. (A, B) β-galactosidase activityinduced by toxR fusions. The results for a six-hour induction in JM101background (A) and for a four-hour induction in DH5α background (B) areshown. The y-axis shows β-galactosidase activity in Miller units; thex-axis, IPTG concentration in micromoles. These results confirm ctxactivation by the toxR-viral fusion proteins and imply viral domaindriven dimerization. (C) Chloramphenicol resistance induced by ToxRfusions. As in FIG. 3, dimerization is demonstrated by resistance tochloramphenicol, reported in μg/mL. "D4" refers to strain DH5α as a hoststrain and the remaining terms are as defined in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

ToxR encodes a transmembrane protein possessing a DNA binding domain atits amino terminus. This intercellular domain has the capacity toactivate expression from the ctx operon. Transactivation depends on thedimerization of toxR protein, which is directed by the protein'sextracellular carboxyl domain. The toxR-ctx system can take the primarysignal of protein-protein association and transduce it into thesecondary message of gene expression. The natural membrane localizationof the toxR protein, which is directed by its transmembrane segment,make it an attractive host for formation of chimera involving membraneproteins. This membrane confinement also has the intriguing feature ofrestricting protein diffusion to a 2-dimensional lipid plane. Placementof the dimerization domain in the extracellular periplasmic space willrender it accessible to exogenous agents that might affect the dimerequilibrium.

Definition of terms

The following definitions apply to the terms as used throughout thisspecification, unless otherwise limited in specific instances.

The terms "dimerization domain" and "region capable of forming a dimer"refer to a polypeptide sequence capable of forming a self-dimer, eitherspontaneously or as a result of ligand binding and the like. Exemplarydimerization domains may be derived from trkC, GCN4, the HSV scaffoldprotein ICP35, HIV integrase, and the like.

The term "reporter gene" refers to any gene whose expression provides ameasurable signal. Exemplary reporter genes include the genes forβ-galactosidase and chloramphenicol transferase. Vadous other reportergenes are well known by those having ordinary skill in the art.

Process of preparation

Gene constructs

The nucleic acids used in the present invention may be prepared byrecombinant nucleic acid methods. See, for example, the recombinant DNAmethods of Nelles , J. Biol. Chem., 262, 10855 (1987). Exemplary strainscomprising such constructs are RFM2016 supE thi Δ(pro-lac) /F'(traD36proAB+lacl^(Q) lacZ ΔM15) /pTOX5 / pCTX4 (ATCC Accession No. 69,403) andRFM2063 supE thi Δ(pro-lac)degP::Tn5/F'(traD36 proAB+lacl^(Q) lacZ ΔM15)/pTOX5trkC12/ pCTX3 (ATCC Accession No. 69,404). ("ATCC" refers to theAmerican Type Culture Collection, 12301 Parklawn Drive, Rockville, Md.20852-1776.

The DNA sequences may be derived from a variety of sources, includinggenomic DNA, subgenomic DNA, cDNA, synthetic DNA, and combinationsthereof. Genomic and cDNA may be obtained in a number of ways. Cellscoding for the desired sequence may be isolated, the genomic DNAfragmented (e.g., by treatment with one or more restrictionendonucleases), and the resulting fragments cloned, identified with aprobe complementary to the desired sequence, and screened for thepresence of a sequence coding for the desired activity.

For cDNA, the cDNA may be cloned and the resulting clone screened with aprobe for cDNA coding for the desired region. Upon isolation of thedesired clone, the cDNA may be manipulated in substantially the samemanner as the genomic DNA.

To express the DNA sequences, transcriptional and translational signalsrecognized by an appropriate host are necessary. It is preferred thatthe toxR gene sequence be operatively linked to an inducible promoter(e.g., the lac promoter), providing a control for the system.

Alternatively, the promoter region from genomic DNA may be obtained inassociation with the DNA sequence for the toxR or other region for thefusion protein. To the extent that the host cells recognize thetranscriptional regulatory and translational initiation signalsassociated with the toxR region, the 5' region adjacent to the codingsequence may be retained and employed for transcriptional andtranslational regulation. This region typically will include thosesequences involved with initiation of transcription and translation,such as the TATA box, capping sequence, CAAT sequence, and the like.Typically, this region will be at least about 150 base pairs long, moretypically about 200 bp, and rarely exceeding about 1 to 2 kb.

The non-coding 3' region may be retained, as well, especially for itstranscriptional termination regulatory sequences, such as the stopsignal and polyadenylated region. In addition, the non-coding 3' regionmay also contain an enhancer. Where the transcriptional terminationsignals are not satisfactorily functional in the host cell, then afunctional 3' region from a different gene may be substituted. In thismethod, the choice of the substituted 3' region would depend upon thecell system chosen for expression.

A wide variety of transcriptional and translational regulatory sequencesmay be employed, depending upon the nature of the host. Thetranscriptional and translational regulatory sequences may be derivedfrom viral sources (e.g., adenovirus, bovine papilloma virus, Simianvirus, and the like) where the regulatory signals are derived from agene that has a high level of expression in the host. Alternatively,promoters from mammalian expression products (e.g., actin, collagen,myosin, and the like) may be employed. Transcriptional initiationregulatory signals may be selected that allow for repression oractivation, so that expression of the genes can be modulated. One suchcontrollable modulation technique is the use of regulatory signals thatare temperature-sensitive, so that expression can be repressed orinitiated by changing the temperature. Another controllable modulationtechnique is the use of regulatory signals that are sensitive to certainchemicals.

To form the toxR or ctx chimeric gene constructs, DNA fragments may beligated in accordance with conventional techniques known in the art.Such techniques include use of restriction enzymes to convertsticky-ended fragments to blunt ends (or vice-versa), polymerases andnucleotides to fill in sticky ends to form blunt ends, alkalinephosphatase to avoid undesired ligations, and ligases to join fragments.

The constructs for toxR and its fusion partner may be joined together toform a single DNA segment or may be maintained as separate segments bythemselves or in conjunction with vectors. The constructs may beintroduced into a cell by transformation in conjunction with a geneallowing for selection where the construct will become integrated intothe host genome. Usually, the construct will be part of a vector havinga replication system recognized by the host cell.

Expression vectors

Expression vehicles for production of the molecules of the inventioninclude plasmids or other vectors. In general, such vectors containcontrol sequences that allow expression in various types of hosts,including but not limited to prokaryotes, yeasts, fungi, plants andhigher eukaryotes. Suitable expression vectors containing the desiredcoding and control sequences may be constructed using standardrecombinant DNA techniques known in the art, many of which are describedin Sambrook, et al., Molecular Cloning; A Laboratory Manual, SecondEdition, Cold Spring Harbor Laboratory, Cold Spring Habor, N.Y. (1989).

An expression vector as contemplated by the present invention is atleast capable of directing the replication of the reporter geneconstruct and the replication and expression of the transmembrane fusionprotein construct. One class of vectors utilizes DNA elements thatprovide autonomously replicating extrachromosomal plasmids derived fromanimal viruses (e.g., bovine papilloma virus, polyomavirus, adenovirus,or SV40). A second class of vectors relies upon the integration of thedesired gene sequences into the host cell chromosome.

Expression vectors useful in the present invention typically contain anorigin of replication, a promoter located 5' to (i.e., upstream of) theDNA sequence to be expressed, and a transcription termination sequence.Suitable origins of replication include, for example, the ColE1, pSC101,SV40 and M13 origins of replication. Suitable termination sequencesinclude, for example, the bovine growth hormone, SV40, lacZ and AcMNPVpolyhedral polyadenylation signals. Suitable promoters include, forexample, the cytomegalovirus promoter, the lacZ promoter, the gal 10promoter and the AcMNPV polyhedral promoter. The promoter sequence mayalso be inducible, to allow modulation of expression (e.g., by thepresence or absence of nutrients or other inducers in the growthmedium). One example is the lac operon obtained from bacteriophagelambda plac5, which can be induced by IPTG.

The expression vectors may also include other regulatory sequences foroptimal expression of the desired product. Such sequences includestability leader sequences, which provide for stability of theexpression product; secretory leader sequences, which provide forsecretion of the expression product; enhancers, which upregulate theexpression of the DNA sequence; and restriction enzyme recognitionsequences, which provide sites for cleavage by restrictionendonucleases. All of these materials are known in the art and amcommercially available. See, for example, Okayama, Mol. Cell. Biol.,3,280 (1983).

A suitable expression vector may also include marking sequences, whichallow phenotypic selection of transformed host cells. Such a marker mayprovide prototrophy to an auxotrophic host, biocide resistance (e.g.,antibiotic resistance) and the like. The selectable marker gene caneither be directly linked to the DNA gene sequences to be expressed, orintroduced into the same cell by co-transfection. Examples of selectablemarkers include neomycin, ampicillin, hygromycin resistance and thelike.

The characteristics of the actual expression vector used must becompatible with the host cell that is to be employed. For a mammalianhost, for example, the expression vector may contain promoters isolatedfrom the genome of mammalian cells, (e.g., mouse metallothionienpromoter), or from viruses that grow in these cells (e.g., vacciniavirus 7.5K promoter).

Suitable commercially available expression vectors into which the DNAsequences of the present invention may be inserted include pSPORT (whichis preferred), pBluescriptIISK, the mammalian expression vectors pcDNAIor pcDNA/Neo, the baculovirus expression vector pBlueBac, theprokaryotic expression vector pcDNAII and the yeast expression vectorpYes2, all of which may be obtained from Invitrogen Corp., San Diego,Calif.

Host Cells

The present invention additionally concerns host cells containingexpression vectors that comprise DNA sequences for the toxR and ctxchimeric gene constructs.

Suitable host cells include both prokaryotic and eukaryotic cells.Suitable prokaryotic host cells include, for example, E. coli strainsHB101, DH5a, XL1 Blue, Y1090 and JM101. Suitable eukaryotic host cellsinclude, for example, Spodoptera frugiperda insect cells, COS-7 cells,human fibroblasts, and Saccharomyces cerevisiae cells.

Mammalian cells that may be useful as hosts include cells of fibroblastorigin (e.g., VERO or CHO-K 1) or lymphoid odgin (e.g., SP2/0-AG14, orP3x63Sg8) or derivatives thereof. Preferred mammalian host cells includeSP2/0 and J558L. Several cell lines secrete urokinase and may be usedfor transfection, such as cultured kidney carcinoma cells and 3T3 cells.Ferrivalo et al., J. Cell Physiol., 121, 363 (1984); Belin et al., EMBOJ., 3, 190 (1984).

Another preferred host is yeast. Yeast provides substantial advantagesin that it can also carry out post-translational peptide modification,including glycosylation. A number of recombinant DNA strategies existthat utilize strong promoter sequences and high copy number of plasmidsthat can be utilized for production of the desired proteins in yeast.Yeast recognizes leader sequences on cloned mammalian gene products andsecretes peptides bearing leader sequences.

Immortalized cells, particularly myeloma or lymphoma cells, are alsosuitable host cells. These cells may be grown in an appropriate nutrientmedium in culture flasks or injected into a synergenic host (e.g., mouseor rat) or an immunodeficient host or host site (e.g., nude mouse orhamster pouch). In particular, the cells may be introduced into theabdominal cavity for production of ascites fluid and harvesting of thechimeric molecule. Alternatively, the cells may be injectedsubcutaneously and the antibodies harvested from the blood of the host.The cells may be used in the same manner as the hybridoma cells. SeeDiamond et al., N. Eng. J. Med., 304, 1344 (1981); MonoclonalAntibodies: Hybridomas--A New Dimension in Biologic Analysis (Kennatt,et al., eds.) Plenum (1980).

Expression vectors may be introduced into host cells by various methodsknown in the art. For example, transfection of host cells withexpression vectors can be carried out by the calcium phosphateprecipitation method. However, other methods for introducing expressionvectors into host cells, for example, electroporation, liposomal fusion,nuclear injection, and viral or phage infection can also be employed.

Host cells containing an expression vector may be identified by one ormore of the following six general approaches: (a) DNA-DNA hybridization;(b) the presence or absence of marker gene functions; (c) assessing thelevel of transcription as measured by the production of mRNA transcriptsencoding the gene constructs in the host cell; (d) detection of the geneproduct immunologically; (e) enzyme assay; and (f) PCR.

In the first approach, the presence of a DNA sequence coding for thegene constructs can be detected by DNA-DNA or RNA-DNA hybridizationusing probes complementary to the DNA sequence.

In the second approach, the recombinant expression vector host systemcan be identified and selected based upon the presence or absence ofcertain marker gene functions (e.g., thymidine kinase activity,resistance to antibiotics, etc.). A marker gene can be placed in thesame plasmid as the fusion protein sequence or reporter construct underthe regulation of the same or a different promoter or reporter.Expression of the marker gene indicates transfection of the vectorhaving the DNA sequence for the fusion protein or reporter gene.

In the third approach, the production of mRNA transcripts encoding thefusion protein or reporter can be assessed by hybridization assays. Forexample, polyadenylated RNA can be isolated and analyzed by Northernblotting or a nuclease protection assay using a probe complementary tothe RNA sequence. Alternatively, the total RNA of the host cell may beextracted and assayed for hybridization to such probes.

In the fourth approach, the expression of the fusion protein or reportercan be assessed immunologically, for example, by immunoblotting withantibody to the fusion protein or one of its regions (Western blotting).Alternatively, this technique could be carried out with the knownepitope of the antibody vadable region.

In the fifth approach, expression of the fusion protein or reporter canbe measured by assaying for its activity (see below).

In the sixth approach, oligonucleotide primers homologous to sequencespresent in the expression system (i.e., expression vector sequences,fusion protein gene sequences, or reporter gene sequences) are used in aPCR to produce a DNA fragment of predicted length, indicatingincorporation of the expression system in the host cell.

The expression vectors and DNA molecules of the present invention mayalso be sequenced. Various sequencing methods are known in the art. See,for example, the dideoxy chain termination method described in Sanger etal., Proc. Natl. Acad. Sci. USA 74, 5463-7 (1977), and the Maxam-Gilbertmethod described in Proc. Natl. Acad. Sci USA 74, 560-4 (1977).

Once an expression vector has been introduced into an appropriate hostcell, the host cell may be cultured under conditions permittingexpression of large amounts of the desired protein.

The fusion protein may be isolated and purified in accordance withconventional conditions, such as extraction, precipitation,chromatography, affinity chromatography, electrophoresis, and the like.The preferred method is affinity chromatography with either the aminoterminal heptapeptide of the fibrin β chain, which binds to theantifibrin site, or benzamidine, which binds to the plasminogenactivator catalytic site.

It should, of course, be understood that not all expression vectors andDNA regulatory sequences will function equally well to express the DNAsequences of the present invention. Neither will all host cells functionequally well with the same expression system. However, one of ordinaryskill in the art may make a selection among expression vectors, DNAregulatory sequences, and host cells using the guidance provided hereinwithout undue experimentation and without departing from the scope andspirit of the present invention.

Preferred Embodiments

Below are detailed descriptions of specific embodiments of the presentinvention. These embodiments are exemplary and serve to illustrate thebroad applicability of the present invention.

The present inventors placed V. choleraee's full length toxR gene behindthe inducible lac promoter of the vector pSPORT. This provides a controlto characterize the behavior of this system in E. coli. To exploit thissystem, the inventors cloned a truncated toxR gene immediately upstreamof a multicloning site in the same parental vector (FIGS. 1 and 2).

TrkC, Fusions

ToxR-trkC fusions were prepared as described in FIG. 4. A large numberof fusions with trkC segments of varying length were screened for toxRfunction (β-galactosidase activity at high IPTG in a host strainharboring the vector pCTX3). Clones with toxR activity were furthercharacterized and the results are shown in FIG. 5.

These fusions demonstrate that the foreign trkC sequences (without theirown transmembrane segment) are able to restore varing levels of toxRfunction, which is fusion-dependent. Deletions that remove only smallportions of trkC provide a fusion protein that demonstrates a toxRsignal at higher levels of expression (high IPTG levels). In addition, anumber of fusions that remove larger segments of trkC show enhanceddimer-forming ability, as demonstrated by toxR activity at low levels ofexpression (without IPTG.) In fact, many of the trkC fusions are morecompetent at ctx activation than is the wild type toxR protein with itsown dimer forming domain. A possible model suggests that sequences inthe wild-type trkC amino terminus obscure a very strong dimer formingcapacity. In this model, NT3 binding in the wild type trkC protein mightcause conformational changes that uncover this strong self-associationdomain.

The modified host cells allow a screen for trkC ligands with NT3-likeactivity, using chloramphenicol resistance or β-glactosidase as anindicator. Ligand activity will be signaled by the appearance ofchloramphenicol resistance or β-glactosidase activity at a level offusion expression below that which gives ligand-independent activity.

Other fusions

In addition to trkC and GCN4-directed toxR association, both the HSVscaffold protein ICP35 and the HIV integrase protein are competent todirect toxR function (see FIG. 6). Other suitable periplasmic bindingregions for this invention include the bacterial chemotaxis receptorstar (aspartate) and tsr (serine), the human insulin receptor and HIVprotease. All such fusions will allow screens to identify inhibitorsspecific to their respective fused dimerization domains.

Materials and method

Strains

Vibrio cholera strain 569B rpf, classical biotype, Inaba serotype(courtesy of H. Smith) (BMS culture collection number SC15307) was usedfor all V. cholerae DNA isolations.

E. coli strain JM101 (Messing 1979) was used in all ctx transactivationexperiments. E. coli strains DH5α(Gibco-BRL) and JM101 were used forgeneral plasmid maintenance and amplification.

Media

Vibrio strains were grown in NB (nutrient broth) supplemented with 0.5%NaCl at 30° C. E. coli strains were grown in LB (Luda-Bertani) broth at37° C. When selection of plasmids was required, ampici lin was used at50 μg/mL and spectinomycin was used at 50 μg/mL. Chloramphenicol wasused at varying concentrations as described in the text.

Genetic techniques

Standard methods were used for the isolation and manipulation of plasmidDNA (Maniatis, T., Fritsch, E. F. & Sambrook, J.(1982) MolecularCloning: A Laborato Manual (Cold Spring Harbor Lab., Plainview, N.Y.).Genomic Vibrio DNA was prepared as described in Clark, J. M. & Switzer,R. L. (1977) Experimental Biochemistry, p. 230 (W. H. Freeman Co.,N.Y.).

Unidirectional deletions were formed using ExoIII, mung bean nuclease,T4 DNA ligase from a unidirectional deletion kit (Stratagene) accordingto protocols supplied by the manufacturer. Briefly, the plasmidpTOX5TRKC was digested with SfiI and NotI restriction endonucleasesprior to the addition of ExoIII. Aliquots were taken at one minuteintervals for 5 minutes from the ExoIII mixture and the plasmid endswere flushed with mung bean nuclease and then ligated to createdeletions into the trkC portion of the toxR-trkC chimera. These plasmidswere transformed into JM101/pCTX3 (J3) and assayed for β-galactosidaseas a measure of their ability to transactivate ctx-LACZ. Active fusionssuch as pTOX5TRKC Δ12 were then sequenced (B. Keinzle) on an ABI DNAsequencer to define the deletion endpoints and verify that an in-framefusion was generated.

DNA was amplified by PCR using a Perkin-Elmer Cetus thermal DNA cycler480 and GeneAmp DNA amplification kit. The reaction mix contained 1XAmplitag Buffer with 1.5 mM magnesium chloride, 200 μM each dNTP, 2.5 UAmplitag polymerase, 1.0 μM5' and 3' primers and 10-100 ng template DNAin 100 μL total volume. In general, the amplification consisted of onecycle of 5 minutes at 94° C, 35 cycles consisting of one minute at 94°C. followed by 2 minutes at 52° C. then 3 minutes at 72° C. andcompleted with a final cycle of 8 minutes at 72° C. PCR products areanalyzed on 0.8% agarose gels and then digested with restrictionendonucleases (at sites engineered into the primers) prior to ligationinto the host vector.

Standard methods were used for SDS-PAGE and immunoblotting of toxRfusion proteins (Maniatis, T., Fritsch, E. F. & Sambrook, J. (1982)Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Lab.,Plainview, N.Y.). Generally, whole cell protein or purified protein wasloaded onto a 10% SDS Daiichi mini gel and electrophoresis was run at 40mA constant current in 1X Laemmli buffer for one hour. Followingelectrophoresis, the proteins were transferred to a supportednitrocellulose membrane in a Hoeffer blotting tank for one to two hoursat 375 mA constant current. Western analysis was conducted using a1:1000 dilution of the primary antibody (rabbit anti-GSTTOX5 polyclonalsera) followed by a second incubation with a 1:3000 dilution of thesecondary antibody (mouse or goat anti-rabbit monoclonal antibodies,BioRad) conjugated to alkaline phosphatase. The membrane was blockedwith 5% powdered skim milk in Tris buffered saline (blotto) before theaddition of the primary antibody and washed three times with blottoafter the addition of each antibody. Antigenic proteins were visualizedusing the western blue stabilized substrate for alkaline phosphatase(Promega).

Plasmid Vectors

pCTX3 and pCTX4. The plasmid pCTX3 was constructed by digesting pMLB1109with PstI and BalI, and isolating the appropriate 5 kb fragment. Thisfragment was then ligated to a 5.7 Kb PstI and SmaI fragment of pMS421.The resulting plasmid, pCTX2, was partially digested with EcoRI todelete the 1.7 Kb lacI^(Q) region. The EcoRI ends from the partial werefilled in with Klenow polymerase and dNTPs, and then ligated to yieldthe vector pSLAC2. The plasmid pCTX3 was constructed by cloning anEcoRI-BamHI flanked PCR fragment of the CTX promoter from p3083 (R.Finklestein) into EcoRI and BamHI digested pSLAC2 to yield the 9.1 Kbplasmid. The plasmid pCTX4 was generated by inserting the BamHI and ClaIdigested fragment of pCaMVCN (Pharmacia) containing the CAT gene intoBamHI and ClaI digested pCTX3.

pTOX3. pTOX4. and pTOX5. The plasmid pTOX3 was constructed by PCRamplifying the toxR coding region with a 5' PstI restriction sitecontaining PCR pdmer and a 3' SfiI restriction site harboring PCR primerusing genomic DNA prepared from the V. cholerae strain 569B rpf obtainedfrom H. Smith. The resulting PCR fragment was cloned into PstI- andSfiI-digested pSPORT1 (BRL).

The plasmid pTOX4 was constructed by PCR amplifying, from genomic V.cholerae DNA, the toxR coding region truncated at base pair 989 of thetoxR reading frame using a PCR primer with a 5' PstI restriction siteand a second primer with both a 3' SfiI restriction sites and stopcodons. The resulting PCR fragment was cloned into PstI- andSfiI-digested pSPORT1 (BRL).

The plasmid pTOX5 was constructed by PCR amplifying a truncated versionof the toxR coding region at base pair 827 of the toxR reading frame ina manner strictly analogous to that used in the pTOX4 construct.

pTOX5ICP35. The plasmid pTOX5ICP35 was developed by digesting pT7ICP35K(I. Deckman) with KpnI and NcoI to yield a fragment containing the HSVICP35 coding region. The NcoI site of this fragment was filled in withTaq polymerase and dNTPs. This modified insert was cloned into KpnI andStuI digested pTOX5 giving a fusion of the ICP35 reading frame alignedwith that of the truncated toxR gene.

oTOX5TRKC+TM. The plasmid pTOX5TRKC+TM was constructed by generating a1.3 kb PCR fragment of the trkC coding region from base pair position 32to position 1391 of the trkC reading frame of pFL 19 (M. Barbacid)containing 3' XbaI and 5' HindIII termini with stop codons derived fromthe PCR primers. This fragment was then cloned (not in-frame) into XbaIand HindIII digested pTOX5 to create a vector that could be used to makeunidirectional deletions of trkC truncated just past the transmembranedomain fused to toxR.

pTOX5TRKC-TM. The plasmid pTOX5TRKC-TM was generated in the same fashionas pTOX5TRKC+TM, but the 1.3 kb PCR-generated (trkC position 32 to 1318)fragment did not contain the trkC transmembrane domain.

pTOX5TRKC-TM del 12. The plasmid pTOX5TRKC-TM Δ12 was created bytreating with ExoIII the SfiI- and NotI-digested pTOX5TRKC-TM. ExoIIItreatment was stopped at one-minute intervals, and mung bean nucleasewas added to flush the ends prior to rejoining the plasmid deletionswith T4 ligase. Deletion number 12 was sequenced and shown to be anin-frame deletion toxR-trkC segment of pTOX5TRKC-TM. The junction of thedeletion contains the amino acid residues TG from the end of thetruncated toxR gene with the intervening linker RSRSE joined to theinitial methionine of the trkC sequence.

pTOX5HIVPROT. The plasmid pTOX5HIVPROT was devised from an SfiI-flanked300-base pair PCR-amplified HIV Protease gene (P-F. Lin) cloned in-frameinto SfiI-digested pTOX5.

pTOX5HIVINT. The plasmid pTOX5HIVINT was formulated by inserting an896-base pair PCR product of the HIV integrase gene, with an SfiI siteat the 5' terminus and XbaI site at the 3' terminus derived from the PCRprimers (P-F. Lin), into SfiI and XbaI-digested pTOX5 to yield anin-frame fusion of the HIV integrase with toxR.

pTOX5GCN4. The plasmid pTOX5LINK was established to allow in-framecloning of the leucine zippers of the yeast vesulatory protein GCN4 withSalI and BamHi termini. This vector was created by digesting pTOX5 withSfiI and BamHI, and cloning in two annealed oligonucleotides (sequences5' to 3' GTCGACG and GATCCGTCGACCTC). The plasmid pTOX5GCN4 wasoriginated by liberating a 129-bp fragment from SalI- and BamHI-digestedpJH370 (J. Hu) and cloning this piece into SalI- and BamHI-digestedpTOX5LINK.

pTOX5LP19, pTOX5LV19, pTOX5LY19, pTOX5LI19, pTOX5LN19, pTOX5LK19. Theplasmids pTOX5LP19, pTOX5LV19, pTOX5LY19, pTOX5LI19, pTOX5LN19,pTOX5LK19 were engineered identically to pTOX5GCN4 but included theSalI-BamHI fragments from pJH524, pJH505, pJH506, pJH518, pJH521, pJH528(J. Hu) respectively.

pGSTTOX5. The plasmid pGSTTOX5 was made by digesting pGEX-3X (Pharmacia)with SmaI and BamHI and cloning in PCR amplified truncated toxR frompTOX5 with compatible ends.

Assays

β-galactosidase assays were performed essentially as described by Menzel(Analytical Biochemistry181, p. 40-50 1989.)

Agar plate assays for activation based on chloramphenicol resistancewere performed with JM101 strains containing the plasmid pCTX4 and anyof the following activating plasmids; pTOX3, pTOX5, pTOX5GCN4,pTOX5ICP35, or pTOX5HIVINT. Overnight cultures were grown in LB with 100μM IPTG. One to two microliters of the overnight culture was spotted onLB-agar plates with 100 μM IPTG and the levels of chloramphenicolindicated in the text. ToxR activity was scored based on the ability togrow on increasing levels of the drug. In the absence of IPTG, allstrains failed to show any chlommphenicol resistance.

The abbreviations used in this specification are defined as follows.

    ______________________________________                                        AcMNPV      Autographa californica multiple nuclear                                       polyhedrosis virus                                                bp          base pairs                                                        CAT         chloramphenicol transferase                                       cDNA        complementary DNA                                                 DNA         deoxyribonucleic acid                                             HIV         human immunodeficiency virus                                      HSV         herpes simplex virus                                              IPTG        isopropylthiogalactoside                                          kb, kbp     kilobase pairs                                                    NT          neurotrophin                                                      PCR         polymerase chain reaction                                         phoA        alkaline phosphatase                                              RNA         ribonucleic acid                                                  ______________________________________                                    

    __________________________________________________________________________    SEQUENCE LISTING                                                              (1) GENERAL INFORMATION:                                                      (iii) NUMBER OF SEQUENCES: 3                                                  (2) INFORMATION FOR SEQ ID NO:1:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 1105 base pairs                                                   (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: double                                                      (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: DNA (genomic)                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                       GTGAGTGAGTGAGTGTTGGGACAGGGACATACTGGGACATTAGATGTTCGGATTAGGACA60                CAACTCAAAAGAGATATCGATGAGTCATATTGGTACTAAATTCATTCTTGCTGAAAAATT120               TACCTTCGATCCCCTAAGCAATACTCTGATTGACAAAGAA GATAGTGAAGAGATCATTCG180              ATTAGGCAGCAACGAAAGCCGAATTCTTTGGCTGCTGGCCCAACGTCCAAACGAGGTAAT240               TTCTCGCAATGATTTGCATGACTTTGTTTGGCGAGAGCAAGGTTTTGAAGTCGATGATTC300               CAGCTTAACCCAAGC CATTTCGACTCTGCGCAAAATGCTCAAAGATTCGACAAAGTCCCC360              ACAATACGTCAAAACGGTTCCGAAGCGCGGTTACCAATTGATCGCCCGAGTGGAAACGGT420               TGAAGAAGAGATGGCTCGCGAAAACGAAGCTGCTCATGACATCTCTCAGCCAGAATCTG T480              CAATGAATACGCAGAATCAAGCAGTGTGCCTTCATCAGCCACTGTAGTGAACACACCGCA540               GCCAGCCAATGTCGTGGCGAATAAATCGGCTCCAAACTTGGGGAATCGACTGTTTATTCT600               GATAGCGGTCTTACTTCCCCTCGCAGTATTACT GCTCACTAACCCAAGCCAATCCAGCTT660              TAAACCCCTAACGGTTGTCGATGGCGTAGCCGTCAATATGCCGAATAACCACCCTGATCT720               TTCAAATTGGCTACCGTCAATCGAACTGTGCGTTAAAAAATACAATGAAAAACATACTGG780               TGGACTCA AGCCGATAGAAGTGATTGCCACTGGTGGACAAAATAACCAGTTAACGCTGAA840              TTACATTCACAGCCCTGAAGTTTCAGGGGAAAACATAACCTTACGCATCGTTGCTAACCC900               TAACGATGCCATCAAAGTGTGTGAGTAGGATCTTGCTATGCAAAATAGACA CATCGCCAT960              GGGTATTCTTCATAGGAAAACTGAAGAGCATCTGATCGACTTCACTATCACAGTTCCCAC1020              GCACAGCAATGATCTGCTGAGCAAACTGATTCAATTTTTCAGCGACGGCTACAGGGTTGT1080              ACCTTGCGGGCCATCTAGGCCTGCA 1105                                                (2) INFORMATION FOR SEQ ID NO:2:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 843 base pairs                                                    (B) TYPE: nucleic acid                                                        (C) STRANDEDNESS: double                                                      (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: DNA (genomic)                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                       TTCCTGCAGTGAGTGA GTGAGTGTTGGGACAGGGAGATACTGGGACATTAGATGTTCGGA60               TTAGGACACAACTCAAAAGAGATATCGATGAGTCATATTGGTACTAAATTCATTCTTGCT120               GAAAAATTTACCTTCGATCCCCTAAGCAATACTCTGATTGACAAAGAAGATAGTGAAGAG 180              ATCATTCGATTAGGCAGCAACGAAAGCCGAATTCTTTGGCTGCTGGCCCAACGTCCAAAC240               GAGGTAATTTCTCGCAATGATTTGCATGACTTTGTTTGGCGAGAGCAAGGTTTTGAAGTC300               GATGATTCCAGCTTAACCCAAGCCATTTCGACTC TGCGCAAAATGCTCAAAGATTCGACA360              AAGTCCCCACAATACGTCAAAACGGTTCCGAAGCGCGGTTACCAATTGATCGCCCGAGTG420               GAAACGGTTGAAGAAGAGATGGCTCGCGAAAACGAAGCTGCTCATGACATCTCTCAGCCA480               GAATCTGTC AATGAATACGCAGAATCAAGCAGTGTGCCTTCATCAGCCACTGTAGTGAAC540              ACACCGCAGCCAGCCAATGTCGTGGCGAATAAATCGGCTCCAAACTTGGGGAATCGACTG600               TTTATTCTGATAGCGGTCTTACTTCCCCTCGCAGTATTACTGCTCACTAACC CAAGCCAA660              TCCAGCTTTAAACCCCTAACGGTTGTCGATGGCGTAGCCGTCAATATGCCGAATAACCAC720               CCTGATCTTTCAAATTGGCTACCGTCAATCGAACTGTGCGTTAAAAAATACAATGAAAAA780               CATACTGGTGGACTCAAGCCGATAGAA GTGATTGCCACTGGTGGCCATCTAGGCCTGCAG840              GAA843                                                                        (2) INFORMATION FOR SEQ ID NO:3:                                              (i) SEQUENCE CHARACTERISTICS:                                                 (A) LENGTH: 145 base pairs                                                    (B) TYPE: nucleic acid                                                        (C ) STRANDEDNESS: double                                                     (D) TOPOLOGY: linear                                                          (ii) MOLECULE TYPE: DNA (genomic)                                             (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                       AACAGAAAATGATAAAAAAGGACTAATAGTATATTTTGATTTTTGATTTTTGATTTTTGA60                TTTTTGATTTTTGATTTTTGATTTTTGATTTCAAATAATACAAATTTATTTACTTATTTA 120              ATTGTTTTGATCAATTATTTTTCTG145                                              

What is claimed is:
 1. A prokaryotic host cell, comprising:(a) atransmembrane fusion protein having(i) a toxR region having a toxRDNA-binding region and a toxR hydrophobic transmembrane region, and (ii)a periplasmic region capable of forming a dimer upon binding of aligand; and (b) a nucleic acid molecule having a reporter geneoperatively linked to the ctx operon;wherein dimer formation is signaledby expression of the reporter gene.
 2. A transmembrane fusion protein,comprising:(a) a toxR region having a toxR DNA-binding region and a toxRhydrophobic transmembrane region, and (b) a region capable of forming adimer in the periplasm of a prokaryotic cell upon binding of a ligand.3. A nucleic acid molecule coding for the transmembrane fusion proteinof claim
 2. 4. An expression vector, comprising the nucleic acid ofclaim
 3. 5. A process for detecting binding of a ligand to a periplasmicregion capable of forming a dimer upon binding of a ligand, whichcomprises:(a) treating a culture of the host cells of claim 1 with aligand, and (b) screening for expression of the reporter gene.
 6. Thehost cell of claim 1, wherein the toxR region has the sequence of SEQ.ID. NO.
 1. 7. The transmembrane fusion protein of claim 2, wherein thetoxR region has the sequence of SEQ. ID. NO.
 1. 8. The nucleic acid ofclaim 3, wherein the toxR region has the sequence of SEQ. ID. NO.
 1. 9.The expression vector of claim 4, wherein the toxR region has thesequence of SEQ. ID. NO.
 1. 10. The process of claim 5, wherein the toxRregion has the sequence of SEQ. ID. NO.
 1. 11. The nucleic acid of claim3, wherein the toxR region has the nucleic acid sequence of SEQ. ID. NO.2.
 12. The expression vector of claim 4, wherein the toxR region has thenucleic acid sequence of SEQ. ID. NO.
 2. 13. The process of claim 5,wherein the toxR region has the nucleic acid sequence of SEQ. ID. NO. 2.14. The host cell of claim 1, designated ATCC 69,403 or ATCC 69,404. 15.The host cell of claim 1, wherein the periplasmic region comprises theligand-binding region of trkC.
 16. The protein of claim 2, wherein theperiplasmic region comprises the ligand-binding region of trkC.
 17. Thenucleic acid of claim 3, wherein the periplasmic region comprises theligand-binding region of trkC.
 18. The expression vector of claim 4,wherein the periplasmic region comprises the ligand-binding region oftrkC.
 19. The process of claim 5, wherein the periplasmic regioncomprises the ligand-binding region of trkC.